Method of and system for recording and reproducing sound



2 Sheets-Sheet l J. R. BALSLEY Filed Nov.

METHOD OF AND SYSTEM FOR RECORDING AND REPRODUCING SOUND mp.. m

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March 3, 1931.

J. R'. BALSLEY Filed NOV. 18,` 1929 2 SheeLs-Sheety 2 liar-neg.

Patented Mar. 3, 1931 UNITED STATES PATENT :OFFICE JAMES R. BALSLEY, -OFBEVERLY HILLS, CALIFORNIA, ASSIGNOR TO FOX V".liILM COR- AIPORAELION, OFHOLLYWOOD, CALIFORNIA, A CORPORATION OF NEW YORK METHOD OF ANDSYSTEM-FOR RECORDING AND REPRODUCING SOUND Application led November 18,1929. Serial No. 407,925.

1'5 illuminating the sound record on the moving film through av narrowslit by means of a beam of light of constant intensity, the portion ofthe light beam transmitted through the film being modulated ,by thesound record thereon, and then striking a photo-electric cell togenerate a modulated electrical current therein, which is finallyreproduced as sound waves by means of a speaker unit.

The width of the light-slit is determined as '25 a compromise betweentwo opposing requirements. The'narrower the slit is made relative to thephysical lengths of the opacity .variations of the sound record, themore accurate is/reproduction; but the lower is vthe light intensity onthe film, and consequently the lower is the level of reproduction. Thusan effort to increase accuracy of reproduction by narrowing the width ofthe slit is defeated beyond a certain point by an accompanying decreasein light intensity, which causes distortion by reason o'fjcausingoperation on the non-linear portion of the-film characteristic curve- Aslit widthof between :0008 and .001

of an inch is now accepted as the best compromise between these opposingfactors in certain present systems; although a greater width would givebetter light intensity on the film, while a narrower width would givebetteraccuracy of reproduction.

Stated in one aspect, is an object of my invention to provide a soundrecording and reproducing system in which increased accuracy inreproduction is gained with the use of a light-slit of a given width.

This object is accomplished, briefly, by

-thatit becomes a step-down frequency congained. In reproduction, theelectrical cur'- rent produced from' the divided-frequency record 1s putthrough a frequency multiplier, properly tuned and balanced to match the-frequency divider, which reproduces the orig-Y f mal microphonecurrent. The'ou-tput of the frequency multiplier is then amplified andreproduced as sound waves' in the usual manner.

The invention will best be understood from the followmg detaileddescription of systems for carrying the invention into effect, referencefor this purpose being had to the accompanying drawi gs, in which:

F1 g. lis a diagram showing a sound recordlng system including step-downfrequency converter or frequency divider;

Fig. 2 1 s a diagram showing a sound record reproducing system includinga step-up frequncy converter or frequency multiplier; an

Fig 3 is a diagram showing a variational type of frequency multiplier.

The frequency divider utilized in the present invention may be of any'type suited to the purpose.- For instance, it may be the type of c1rcu1tincluded in the system yillustrated in Fi 1 of the drawings, whichcircuit is descri ed in an article in the Proceedings 'of the Instituteof Radio Engineers for February 1929, on pages 255-259.

As described in the article referred to, this u I circuit is similar tothe Abraham-Bloch multivibrator circuit, modified in such a' way verter.The multivibrator consistsessentially of an aperiodic circuit in whichoscillations are sustained'by a ltriode excited by a second triode whichprovides the proper phase relation for maintenance.y -In its usual 100form the multivibrator acts as source of high frequency oscillationscontrolled by a low frequency injected oscillation. `In its usual formit therefore constitutes a frequency multiplier or step-up frequencyconverter. The circuit in this usual form may be used in reproductionaccording to the present invention, as will hereinafter be explained.

As arranged in Fig. 1, however, the circuit is controlled at afundamental frequency which is an integral submultiple of the injectedcontrol frequency, thus making it a step-down frequency converter.

Referring now to Fig. 1, the letter M indicates a microphone which isconnected to an amplifier 10, conventionally illustrated as being of thevacuum tube type. The output leads 11 and'12 of amplifier 10 areconnected v Ainto the plate circuit 13 of a frequency divider 14 throughthee medium of a vacuum tube 15, which tube acts as a one way relay toisolate amplifier 10 from reactions due to the frequency divider. Forcertain separated ranges of values for condensers 16, the fundamentalfrequency of the frequency divider assumes values which are submultiplesof the frequencies injected through leads 11 and 12. The orde;` ofcontrol of the frequency divider, or', in other words, the ratio of theinjected frequency to the oscillation frequency of the frequency dividercircuit, vis .dependent upon the various constants of the frequencydivider circuit; and afrequency division by a relatively small number orb a number as high as 100 or more is readily o tainable by varying theseconstants. The output leads 20 and 21 of the frequency divider, whichcarry an electrical current of stepped-down frequency, may be connectedto a vacuum tube amplifier 24, and the output of amplifier 24 isconducted by means of leads 25 and 26 to a recording lamp 27. Alight-slit 28 is positioned before lamp 27 and a lens 29 casts an imageof the illuminated slit 28 upon a moving film 30 to print a record-31thereon of the divided-frequency current. It will be obvious that theindividual opacity variations of record 31 will be increased in physicallength overA Vwhat they would be in usual recording, .by a

factor-equal to thestep-down ratio of the frequency divider.

In reproduction, the film 30 within its divided-frequency record 31 isoperatedA in the usual manner in conjunction with a light-slit, toproduce an electrical current wave at the reduced frequency, and thiselectrical current wave is thereafter stepped-up or multiplied infrequency by the factor originally utilized in frequency division,thereby reproducing the original sound wave current which may bereproduced as audible sound by meansof a usual speaker unit.

. For this purpose, the system of Fig. 2 may be utilized. The film 30with the divided-fre.- quency record 31 is moved before a light-slit-32, which slit is illuminatedk by means-of an quency multiplier 41,which, in the system illustrated in Fig. 2, embodies' an electricalcircuit of the multivibrator type, similar to thatutilized inthefrequency divider of the recording system, By properly balancing theconstants of the frequency multiplier circuit 42, osciillations may beset up therein at a stepped-up frequency; and by carefully oadjustingthe frequenc divider and frequency multiplier with re erence to eachother, the output frequency of the frequency multiplier may be perfectlymatched with" the frequency of the original input current injected intothe frequency divider."

The output leads 45 and 46 ofthe frequency multiplier may be connectedto a vacuum tube amplifier 4 7, and the output leads 48 and 49 thereofare connected to a speaker unit 50.

-The general operation of the system may briefly be summed up asfollows: The sound wave current produced by the 'microphone is dividedin frequency by a selected number, and the divided-frequency electricalcurrent thereby produced is recorded upon a photographic film. Thephysical lengths of the individual opacity variations ofthe photographicrecord are then obviously increased by the factor of frequency divisionover what they would have been had the original sound v wave currentbeen recorded directly, and they are also increased by this same factorrelative to the light-slit used in reproduction. And since, ashas-previously been 'explained, the accuracy of reproduction of aphotographic record depends upon the physical lengths of the individualopacity variation relative to the width of the light-slit, thedivided-frequency record will be reproduced with an accuracy increasedby the same factor over the highest accuracy possible in reproduction ofusual full-Wave sound records. A divided-frequency electrical current isthen produced from the record with the use of a light-slit in the mannerexplained, and this current is then multiplied in frequency to match theoriginal sound Awave current injected into the frequency divider, themultiplied-frequency 4current being linally reproduced as audible soundin the forth merely for the purpose of simplicity of explanation.

Another form of frequency multiplier which may be mentioned isillustrated in Fig. 3. The frequency multiplier of this illustration,which will be understood to be adapted to replace the frequencymultiplier in the reproducing system of Fig. 2, is a well knownarrangement for doubling frequency, and will therefore be herein-butbriefly described. The input circuit 60 leads to the series connectedwindings of a pair of transformers\61 and 62, each equipped with atertiary circuit 63 applied with current from a common battery B. Thesteady current through these windings is adjusted to bring thetransformer fluxes just to the point where saturation occurs. The twoprimary windings in series being connected to a pulsating current,during the'positive half cycles the flux in transformer 6 1 (wherein thm. f. of the primary winding assists the direct current winding), willchange very little, while the flux in winding 62 will decreaseconsiderably, since in this case the primarym. m. f. opposes the m. in.f. of the direct current winding. On the negative half cycle, the,reverse is true. The iux of one transformerthus has a large variationduring one alteration while kthe flux of the other transformer changesonly slightly; and on the next alteration these conditions are reversed.The

l electromotive forces induced in the respec- It will be recognized thatthe illustrative u system specifically described herein is capable ofconsiderable modification and rear-V rangement without departing fromthe spirit and scope of my invention; and it is therefore j tobeunderstood that the following claims embrace all such modificationsand equivalent .arrangements as may fairly be construed to fall withinthe scope of\my invention.

1. The method of recording and reproducing sound, that includestransforming sound waves tinto electrical current Waves correspondingthereto, producing therefrom' an electrical current'wave of stepped downfref quency, makingl a record of said current Wave 'of stepped downfrequency, reproducing from said record its recorded electrical currentwave, producing therefrom an electrical current wave stepped up infrequency to match the Original sound Wave frequency and reproducingsaid last mentioned current wave as an audible sound wave.

2. The method of recording and reproducing sound, that includestransforming sound Waves into electrical current waves corresaid currentwave of stepped down frequency, reproducing from said record itsrecorded electrical current wave, producing therefrom an electricalcurrent wave stepped up in frequency to match theoriginal sound wavefrequency, and reproducin said last mentioned current wave as an au iblesound wave. 3. The method of recording and reproducing sound,` thatincludes transforming sound waves into electrical current Wavescorresponding thereto, dividing the frequency of said electrical currentwave by a given number, making a record of .the di-vided-frequencycurrent wave, reproducing from said record its recordeddivided-frequency electrical current wave, multiplying the fre-- quencyof said divided-frequency current Wave by the number originally Vused infre-y ing the frequency of said divided-frequency* current wave by thenumber originally used 1n frequency dlvision, and reproducing saidmultiplied-frequency current wave as audible sound.

5. In a sound recording and reproducing system, the combination of meansfor transforming sound waves into electrical current waves, means fordividing the frequency of said current waves` by a given number,recording means for making a record of the electrical current wavesoproduced, means for reproducing the current waves recorded on saidrecord, means formultiplying the frequency of 'said current waves by thenumber used in original frequency" division, and means for reproducingsound waves from the multiplied-frequency current wave so produced. 6.In a souiid recording and reproducing system, the combination of meansfor transforming sound waves into electrical current waves, means fordividing the frequency of said current waves by a given number,photographic recordin means for making photographic record o Vtheelectrical current wave so produced, meansfor reproducing the currentwaves recorded on said photographic record, means for multiplying thefrequency of said current waves by the number used in original frequencydivision, and means for reproducing sound waves from the multi-Aplied-frequency current wave so produced.

In witness that I claim the oregoin I have hereunto subscribed my namethis 1 th day of September, 1929.

JAMES R. BALSLEY.

